In general, multiuser detection deals with demodulation of the mutually interfering digital streams of information that occur in areas such as wireless communications, high-speed data transmission, DSL, satellite communication, digital television, and magnetic recording. The multiuser detection is also being currently investigated for demodulation in low-power inter-chip and intra-chip communication.
Multiuser detection encompasses technologies devoted to joint detection of interfering signals received over a shared channel Mutual interference is unavoidable in modern spectrally efficient wireless systems: even when using orthogonal multiplexing systems such as TDMA, synchronous CDMA or OFDMA, multiuser interference originates from channel distortion and from out-of-cell interference. In addition, in multi-antenna (MIMO) systems, the digitally modulated streams emanating from different antennas interfere at the receiver, and the MIMO receiver uses multiuser detection techniques to separate them. By exploiting the structure of the interfering signals, multiuser detection can increase spectral efficiency, receiver sensitivity, and the number of users the system can sustain.
For example, consider a communication system where users (transmitters) are in communication with a single receiver, such as a base station and/or an access point. The users transmit in the same frequency band and potentially in the same time slot, and are equipped with spreading codes so that the receiver can separate their transmissions (i.e., detect symbols they transmit). The separation is done by means of filter bank in the receiver's front end. The filter bank include correlators, each correlator correlates the received signal with a certain code, specific to such a correlator. In a conventional multi-user detection system, the number of correlators in the filter bank is equal to the number of spreading codes used by transmitters, so that each correlator is associated with one transmitter. In addition, the spreading codes in the conventional system are designed to be orthogonal or close to be orthogonal.
However, the number of orthogonal spreading codes is limited, and can be insufficient for a multiuser communication system, such as IoT systems with hundreds or even thousands of users/transmitters. Such a deficiency leads to the usage of non-orthogonal codes, which in turn leads to packet collisions and to challenging packet recovery problems, see, e.g., U.S Publication 2017-0244815.